1. Technical Field
The present invention relates generally to a hydraulically actuated clutch assembly in a power transmission, and more particularly, to an apparatus for controlling the flow of a cooling fluid to a fluid bath or "wet" clutch assembly to maximize flow during movement of the clutch assembly components between engaged and disengaged positions and minimize flow at other times.
2. Description of the Prior Art
An industrial vehicle, such as a lift truck or the like, typically includes a fluid-pressure actuated clutch in a power transmission device for driving the vehicle. During operation it is often necessary to hold the vehicle stationary while a fork the lift truck is operated at a speed corresponding to full engine speed without shifting the transmission to the neutral position. It is desirable, in such circumstances, to be able to precisely move or "inch" the vehicle in either the forward or reverse direction in order to achieve accurate positioning of the lift truck and fork in the handling of materials.
Various valve structures have been provided to affect improved gradual engagement and disengagement of the vehicle clutch assembly during "inching" maneuvers in conjunction with the application and release of the vehicle brakes. In such valve structures, the operation of the valve may be either hydraulic or mechanical. When the brakes are actuated, the valve shifts to a position partially interrupting fluid flow from a clutch control valve to effect a controlled partial deactuation of a clutch assembly by maintaining a relatively low fluid pressure.
Fluid bath or "wet" clutch assemblies are also used in the power transmissions to mechanically couple a rotatable drive member on a shaft with a relatively rotatable driven member on the shaft. It is possible that several "wet" clutch assemblies may be used in combination to establish different drive connections throughout the transmission to provide various gear ratios in operation. Each clutch assembly includes a plurality of axially adjacent and alternating friction disks and clutch plates. A group of the friction disks or clutch plates rotate with the drive member and a remaining group of friction disks or clutch plates rotate with the driven member. When the friction disks and clutch plates are axially pressed together into an engaged position, the drive member and driven member are coupled to rotate together. When the friction disks and clutch plates are in a disengaged position, not pressed together, the driven member is free to rotate relative to the drive member and the shaft.
A hydraulic actuator is typically used to move the friction disks and clutch plates of the clutch assembly between the engaged and disengaged positions. The actuator includes a piston-like member located in a fluid chamber usually defined by at least a portion of the drive member. When pressurized fluid is conducted to the chamber, the actuator applies an axial compressive force against the friction disks and clutch plates to establish a mechanical drive connection between the drive and driven members. During movement of the clutch assembly between the fully engaged and fully disengaged conditions or during "inching", the friction disks and clutch plates may slip relative to each other. Heat is generated by the slippage between the friction disks and clutch plates during this movement of the clutch assembly. Too much heat can damage the clutch assembly, thus, involving time consuming and costly repairs. Thus, it is very desirable to remove this heat during actuation movement of the clutch assembly.
To dissipate the heat generated, it is known to bathe the clutch assembly with a cooling fluid. The cooling fluid dissipates the heat developed by the relative slipping between the friction disks and clutch plates. However, constant bathing of the clutch area is not needed because the slipping and heat generation occurs only during movement of the clutch assembly between the engaged and disengaged positions. Flooding the clutch area with cooling fluid also creates viscous drag on the clutch assembly. This viscous drag causes a loss in horsepower and produces additional heat in the clutch assembly at a time when cooling fluid is not necessary.